Conventionally, there have been well known linear motion guide units of the sort that more than one rolling element is allowed to roll through a recirculation circuit made in a slider. In the prior linear motion guide units of the type constructed as stated earlier, as shown in FIG. 14, a carriage 3 of the slider is generally made thereon with a raceway groove 7 including a raceway surface for the provision of a load race 13. The raceway groove 7 is moreover is made in a crowning profile 43 extending across an ingress area 39, which slopes gently towards any one of forward and aft ends of the carriage 3 so as to allow the rolling elements rolling through with smoothness in and out of the load race 13. With the linear motion guide unit constructed as stated just above, the load race 13 coming into rolling-contact with the rolling element, when viewed in lengthwise transverse section, composed of forward and aft ingress areas 39 and a major load area 38 continuing the ingress areas 39 to carry substantially the bearing load thereon. The slider is comprised of the carriage 3 having thereon the raceway groove 7 to provide the load race 13, and forward and aft end caps 4 installed on the lengthwise ends of the carriage 3, one to each end, which are made therein with turnarounds 12. The crowning profile 43 is made in a gently linear slope towards any associated end of the carriage 3. At a junction of the carriage 3 with the end caps 4 where any one of the load race 13 and the turnarounds 12 transfers to the other, moreover, the carriage 3 is cut away slantwise at forward and aft ends 8 to make beveled edges or chamfered edges 44, which extend straight from the crowning profile 43 towards their associated inside curved surfaces 27 of the end caps 4. In other words, the crowning profile 43 is cut away at the forward and aft ends thereof to provide the chamfered edges 44.
Another linear motion guide unit is disclosed in, for example, Japanese Patent Laid-Open No. 2003-322150, in which the slider is made with linear crowning profiles to get reducing variations in rolling-contact load component in traveling direction of the slider, which might occur depending on traveling of the rolling elements, thereby rendering deviations in sliding resistance less. With the prior linear motion guide unit recited above, linear crowning profiles are provided at axially opposite end areas of the slider in a way that the maximum total load exerted by the rolling elements in linear traveling direction is made less than the peak of load exerted by just one rolling element in linear traveling direction to any one linear crowning profile of the slider. Moreover, a prestressed pressure (δ) in the prior linear motion guide unit is generally determined at δ≧0.002Da. The prior linear motion guide unit as stated earlier, further, is shown as being made at the forward and aft ends thereof with arced crowning profiles identical with one another in radius (R).
The design considerations to form the crowning profile in the rolling linear motion guide unit are disclosed in, for example Japanese Patent Laid-Open No. 2003-35314. On prior designing recited here, the crowning profile is made to keep the stiffness substantially constant in at least any one of vertical, horizontal and rolling directions of the slider, even though the rolling elements travel in their axial direction between the confronting raceway grooves. The slider is also designed to keep the stiffness substantially contact in any one or both of pitching and yawing directions. The prior design consideration is envisaged making the crowning profile most suitable for reducing vibration that might occur due to travel of the rolling element, thereby improving the accuracy in sliding motion. With the prior design considerations for the provision of the rolling motion guide unit that is kept constant in stiffness, there is proposed the ideal crowning profile that is designed according to any of curves given by power-series function, composite curves, curves given by method of least squares, and so on.
A further another prior guide unit is disclosed in, for example Japanese Patent Laid-Open No. 2002-155936, in which the non-loaded area is connected with the associated load area in a way any inside edge of the turnaround passage doesn't get prominent into the raceway, which might otherwise cause any obstacle against the rolling elements during their transfer from the non-loaded area to the load area. This prior guide unit is envisaged making it possible to render the sliding resistance and noise less even when the slider travels with high velocity relatively to the guide rail, thereby achieving high-speed traveling of the slider. With the prior guide unit recited jut above, the load raceway groove extending over the load area is worked with crowning operation at the lengthwise end zones thereof to have the crowning profile that is made in a way the load raceway groove recedes gradually away from the non-loaded raceway groove as nearing the associated turnaround passage. Thus, the load raceway surface is set back at the lengthwise end thereof below the inside edge of the turnaround passage. This crowning profile would make it possible to keep certainly the load raceway groove against prominent above the inside edge of the turnaround passage towards the guide rail, regardless of whether there are any molding error in the turnaround passage and any machining error in the load raceway groove. The setback is selected to be a matter of 5% relatively to the diametral dimension of the rolling element.
Meanwhile, with the prior linear motion guide units constructed as stated earlier, the crowning profile 43 and the beveled edge 44 made on the carriage 3 have to be performed using additional operation including material removing operation and so on, apart from the working operation to make the raceway groove 7 for the provision of the load race. This means that the crowning profile 43 made on the carriage 3 can vary widely in length (LA) as well as in depth (HA) from product to product. Moreover, a boundary between the crowning profile 43 and the beveled edge 44 gets to be a sharp corner 46. This means that both the crowning profile 43 and the beveled edge 44 must undergo any fine-tuning to ensure high-speed traveling accuracy of the rolling elements. With the production of the prior linear motion guide units constructed as stated earlier, moreover, the repeatedly simulated experimental results must be conducted as often as needed to determine the crowning profile in length (LA) and in depth (HA).
In the prior linear motion guide units constructed as stated earlier, the crowning length (LA) and crowning depth (HA) can't be disclosed as any preselected length and/or depth, instead found relying on design restrictions. In addition, the raceway groove is cut to have a modified semi-circular arc in transverse section, which will come into rolling contact with the rolling element at only two points of the rolling element.
For producing the linear motion guide units needing the design considerations as stated earlier, the crowning profile would be worked with recently advanced machining technology including computer numerical control (NC) machines. The crowning profile is however sophisticated in construction, involving complex computer modeling recipes and highly refined skill for machining operation. Moreover, the rolling linear motion guide unit constructed as stated earlier has the raceway groove that is made to have a modified semi-circular arc in transverse section, which will come into rolling contact with the rolling element at only two points of the rolling element.
With the prior guide unit constructed as recited above, the crowning profile is made in a straight slope surface ending in a depth to provide a setback below the associated lengthwise edge of the carriage. Thus, the crowning profile is formed slantwise with a considerably large sloping angle, resulting in making a stepwise discontinuity between the carriage and the inside edge of the turnaround passage. After attempts to work out the crowning depth at the lengthwise end of the carriage with the premise that the inside curved surface of the turnaround passage is usually set within the tolerance of ±0.05 mm in accurate position alignment with the load raceway groove, it has been found that the dimension given by subtracting the entire distance of the load raceway groove and the non-loaded raceway groove from the overall length of the inside curved surface of the turnaround passage and the raceway groove on the guide rail would result in a matter of 0.1 mm. If the rolling element were, for example 4.7625 mm in diameter, the stepwise difference at the end of the carriage would reach 0.338 mm because of the addition of 0.238 mm that corresponds to 5% of the diameter of rolling element. This means that the crowning profile is too steep to deal with highly accurate requirements.
Thus, it is now needed to overcome the disadvantages as stated earlier in the linear motion guide unit of standard design specifications rather than the linear motion guide unit customized to suite special design specifications. Moreover, it is desired to develop the linear motion guide unit that is compliant to the high accuracy at recent high-speed traveling operation, and better long-lasting durability even under conditions of usage in high-speed, high-acceleration/high-deceleration operation. That is to say, the linear motion guide unit is recently expected to well cope with the high precision and high durability with respect to the race, and especially serve the high durability with keeping high accuracy even under high-speed, high-acceleration/high-deceleration operational environment.